TCAD Device Simulation of All-Polymer Solar Cells for Indoor Applications: Potential for Tandem vs. Single Junction Cells

The utilization of indoor photovoltaics makes it feasible to harvest energy from artificial light sources. Although single-junction indoor photovoltaics have demonstrated exceptional efficacy when using LED lighting, there is still a need for more comprehensive testing of tandem structures. Herein, the first systematic TCAD simulation study on the potential for tandem all-polymer solar cells (all-PSCs) for indoor applications is provided. The presented all-PSCs are based on experimental work in which the top wide bandgap subcell comprises a polymer blend PM7:PIDT, while the bottom narrow bandgap subcell has a polymer blend PM6:PY-IT. Standalone and tandem cells are simulated under AM1.5G solar radiation, and the simulation results are compared with measurements to calibrate the physical models and material parameters revealing PCE values of 10.11%, 16.50%, and 17.58% for the front, rear, and tandem cells, respectively. Next, we assessed the performance characteristics of the three cells under a white LED environment for different color temperatures and light intensities. The results showed a superior performance of the front cell, while a deterioration in the performance was observed for the tandem cell, reflecting in a lower PCE of 16.22% at a color temperature of 2900 K. Thus, an optimized tandem for outdoor applications was not suitable for indoor conditions. In order to alleviate this issue, we propose designing the tandem for indoor lightening by an appropriate choice of thicknesses of the top and bottom absorber layers in order to achieve the current matching point. Reducing the top absorber thickness while slightly increasing the bottom thickness resulted in a higher PCE of 27.80% at 2900 K.


Introduction
Photovoltaic (PV) technologies are poised to play a vital role in the transition to more sustainable energy solutions [1], with crystalline Si currently serving as the leading semiconducting material. Different Si-based structures were researched, where some have relatively higher costs [2,3], while others have lower efficiencies [4][5][6][7]. On the other hand, thin-film solar cells represent the latest advancement in PV technology. An ideal photoactive material for use in solar cells must meet several key requirements, including high optical absorption, long diffusion lengths, and the ability to form a decent electronic junction (homo/hetero/Schottky) with appropriately well-matched materials [8][9][10]. In this context, all-polymer solar cells (all-PSCs) represent a highly competitive PV technology with high efficiency and low cost. These solar cells possess a high absorption coefficient, enabling less material usage compared to c-Si technology. All-PSCs offer priority in terms of the cost of production, combining lower energy requirements for manufacturing with superb thermal stability and mechanical flexibility [11][12][13]. In recent years, the power conversion efficiency (PCE) of all-PSCs has progressively boosted from less than 10% to about 18% [14][15][16].
Research efforts are currently concentrating on multi-junction solar cells, where thermalization losses can be minimized, assisting higher PCEs of solar cells. In this regard, the double-junction (tandem) solar cell is the simplest device model, which presents a

Solar Cell Configurations and Material Parameters
The tandem cell under investigation comprises all-PSCs for the front and the rear subcells. It is based on an experimental cell as published in [16]. The traditional device structure of ITO/PEDOT:PSS/Blend (PM7:PIDT for the top cell and PM6:PY-IT for the bottom cell)/PDINN/Ag was used to fabricate the standalone all-PSCs, where ITO-coated glass substrates were utilized. The device structures and energy level diagrams of the standalone all-PSCs with PM7:PIDT top cell and PM6:PY-IT bottom cell are shown in Figure 1a,b, respectively. The E HOMO and E LUMO energy levels of the two PIDT and PY-IT were determined to be −5.80/−5.68 and −3.74/−3.77 eV, respectively, through electrochemical cyclic voltammetry, while the mobilities were measured using the photo-CELIV technique [16]. Further, the absorption spectra of both PM7:PIDT and PM6:PYIT blend active layers showed a good complementary in the 300-900 nm wavelength band. Some typical parameters, extracted from the literature as indicated, of the different layers of all-PSCs are listed in Table 1 [16,[29][30][31]. PEDOT:PSS and PDINN (or C60) act as a hole transport layer (HTL) and electron transport layer (ETL), respectively. The work functions of both ITO and Ag contacts are 4.7 and 3.72 eV, respectively [30,31]. Regarding the tandem device, the ETL of the front subcell is taken to be C60 instead of PDINN utilized in the standalone cells. The energy band profile of the tandem is displayed in Figure 1c, where BPhen serves as an interlayer.

Solar Cell Configurations and Material Parameters
The tandem cell under investigation comprises all-PSCs for the front and the rear subcells. It is based on an experimental cell as published in [16]. The traditional device structure of ITO/PEDOT:PSS/Blend (PM7:PIDT for the top cell and PM6:PY-IT for the bottom cell)/PDINN/Ag was used to fabricate the standalone all-PSCs, where ITO-coated glass substrates were utilized. The device structures and energy level diagrams of the standalone all-PSCs with PM7:PIDT top cell and PM6:PY-IT bottom cell are shown in Figure 1a,b, respectively. The EHOMO and ELUMO energy levels of the two PIDT and PY-IT were determined to be −5.80/−5.68 and −3.74/−3.77 eV, respectively, through electrochemical cyclic voltammetry, while the mobilities were measured using the photo-CELIV technique [16]. Further, the absorption spectra of both PM7:PIDT and PM6:PYIT blend active layers showed a good complementary in the 300-900 nm wavelength band. Some typical parameters, extracted from the literature as indicated, of the different layers of all-PSCs are listed in Table 1 [16,[29][30][31]. PEDOT:PSS and PDINN (or C60) act as a hole transport layer (HTL) and electron transport layer (ETL), respectively. The work functions of both ITO and Ag contacts are 4.7 and 3.72 eV, respectively [30,31]. Regarding the tandem device, the ETL of the front subcell is taken to be C60 instead of PDINN utilized in the standalone cells. The energy band profile of the tandem is displayed in Figure 1c, where BPhen serves as an interlayer.

Simulation Methodology
For the current simulation study, the design and evaluation of the tandem as well as the standalone all-PSCs were carried out using the Silavco Atlas simulator. This TCAD software is widely used in modeling and simulating thin-film solar cells [32]. In this simulator, Poisson's equation is solved concurrently with electron and hole continuity equations. Using the drift-diffusion transport model, the electron and hole current densities are then determined. It is important to note that the recombination mechanisms considered include Shockley-Read-Hall (SRH), direct band-to-band, and Auger recombination. Additionally, the definitions of defect properties such as trap energy levels and densities can be incorporated in the bulk of the material or at the interfacial zones. The boundary conditions of front and back contacts are set to accomplish thermionic emission, with identified surface recombination velocities for both electrons and holes. Further, a lumped resistance is included as an interlayer instead of the BPhen interlayer in the following simulation, which allows the current to flow freely between the two subcells of the tandem device without significant impedance. Regarding the optical simulation, the absorption coefficients are extracted from experimental data and are then fine-tuned to match the measured external quantum efficiency (EQE), resulting in a correct estimation for the short circuit current density. A general outline of the simulation process and the specific steps can be summarized in a flowchart as can be depicted in Figure 2.

Simulation Methodology
For the current simulation study, the design and evaluation of the tandem as well as the standalone all-PSCs were carried out using the Silavco Atlas simulator. This TCAD software is widely used in modeling and simulating thin-film solar cells [32]. In this simulator, Poisson's equation is solved concurrently with electron and hole continuity equations. Using the drift-diffusion transport model, the electron and hole current densities are then determined. It is important to note that the recombination mechanisms considered include Shockley-Read-Hall (SRH), direct band-to-band, and Auger recombination. Additionally, the definitions of defect properties such as trap energy levels and densities can be incorporated in the bulk of the material or at the interfacial zones. The boundary conditions of front and back contacts are set to accomplish thermionic emission, with identified surface recombination velocities for both electrons and holes. Further, a lumped resistance is included as an interlayer instead of the BPhen interlayer in the following simulation, which allows the current to flow freely between the two subcells of the tandem device without significant impedance. Regarding the optical simulation, the absorption coefficients are extracted from experimental data and are then fine-tuned to match the measured external quantum efficiency (EQE), resulting in a correct estimation for the short circuit current density. A general outline of the simulation process and the specific steps can be summarized in a flowchart as can be depicted in Figure 2.

Calibration of Standalone and Tandem Cells
To ensure the accuracy of our simulation, we performed a calibration step to validate the model implemented in the Atlas simulator. The J-V (current density-voltage) curves for the top and bottom subcells, obtained from the simulation and experimental data extracted from [16], are presented in Figure 3a, along with the simulated EQEs presented in Figure 3b. The PV parameters obtained from the simulation are listed in Table 2, where J SC is the short-circuit current density and V OC is the open-circuit voltage, while FF represents the fill factor. As indicated in the table, the output parameters matched well with the experimental results. The results revealed that the front cell had a lower PCE than that of the rear cell for these designed cells. Furthermore, the tandem cell was simulated using the same layers encountered in the experimental work. The J-V characteristics are exhibited in Figure 3c, while the PV factors are demonstrated in Table 2, signifying a decent fit for the simulation compared to the measurements and emphasizing the validation of the TCAD models. It should be pointed out here that the optical bandgaps of the blends were different from the electrical bandgap values (that can be deduced from Table 1). From the EQE curves, the estimated values of the optical energy gaps for the front and rear blends were 1.55 eV and 1.3 eV, respectively. Therefore, the two blends were complementary and can cover a wide range of the spectrum.

Calibration of Standalone and Tandem Cells
To ensure the accuracy of our simulation, we performed a calibration step to validate the model implemented in the Atlas simulator. The J-V (current density-voltage) curves for the top and bottom subcells, obtained from the simulation and experimental data extracted from [16], are presented in Figure 3a, along with the simulated EQEs presented in Figure 3b. The PV parameters obtained from the simulation are listed in Table 2, where JSC is the short-circuit current density and VOC is the open-circuit voltage, while FF represents the fill factor. As indicated in the table, the output parameters matched well with the experimental results. The results revealed that the front cell had a lower PCE than that of the rear cell for these designed cells. Furthermore, the tandem cell was simulated using the same layers encountered in the experimental work. The J-V characteristics are exhibited in Figure 3c, while the PV factors are demonstrated in Table 2, signifying a decent fit for the simulation compared to the measurements and emphasizing the validation of the TCAD models. It should be pointed out here that the optical bandgaps of the blends were different from the electrical bandgap values (that can be deduced from Table 1). From the EQE curves, the estimated values of the optical energy gaps for the front and rear blends were 1.55 eV and 1.3 eV, respectively. Therefore, the two blends were complementary and can cover a wide range of the spectrum.  Table 2. Key PV metrics extracted from TCAD and experimental illuminated J-V characteristics for both front and rear subcells. All experimental data were extracted from [16].

Indoor Light LED Characteristics
The emission spectra of commonly used indoor light sources, including fluorescent lamps and LEDs, typically cover a range of 400 to 700 nm with intensities below 1 mW/cm 2 [33]. These spectra are much narrower and have weaker intensities than the standard AM1.5G spectrum. This means that PV cells developed for converting sunlight may not be optimally suited for indoor applications. In addition, the solar cell response according to indoor light sources is completely different from that under AM1.5G illumination.  Table 2. Key PV metrics extracted from TCAD and experimental illuminated J-V characteristics for both front and rear subcells. All experimental data were extracted from [16].

Indoor Light LED Characteristics
The emission spectra of commonly used indoor light sources, including fluorescent lamps and LEDs, typically cover a range of 400 to 700 nm with intensities below 1 mW/cm 2 [33]. These spectra are much narrower and have weaker intensities than the standard AM1.5G spectrum. This means that PV cells developed for converting sunlight may not be optimally suited for indoor applications. In addition, the solar cell response according to indoor light sources is completely different from that under AM1.5G illumination. Thus, to assess the performance of all-PSCs presented herein under the influence of indoor light source, we used the spectra of artificial white LEDs, which consist of a sharp blue emission from GaN. The illuminance of the incident light can be adjusted from 200 to 10,000 lx (which corresponds to 57.9-2895 µW/cm 2 ) [33]. Figure 4 shows the photon flux as a function of wavelength for incident LED light sources with varying color temperatures, as extracted from [33]. A temperature of 7500 K corresponded to a cool white LED, where the photon flux peaked at around 450 nm in the blue part of the spectrum. The flux gradually decreased as the wavelength increased towards the red part of the spectrum. For a color temperature of 5300 K, the photon flux had another slight peak at around 630 nm, which was in the green-yellow part of the spectrum. A warm white LED occurred for 2900 K, wherein the photon flux peaked at around 630 nm, which was in the orange-red part of the spectrum. The flux gradually decreased as the wavelength increased towards the blue part of the spectrum.
Thus, to assess the performance of all-PSCs presented herein under the light source, we used the spectra of artificial white LEDs, which con emission from GaN. The illuminance of the incident light can be ad 10,000 lx (which corresponds to 57.9-2895 µW/cm 2 ) [33]. Figure 4 sho as a function of wavelength for incident LED light sources with vary tures, as extracted from [33]. A temperature of 7500 K corresponded t where the photon flux peaked at around 450 nm in the blue part of the gradually decreased as the wavelength increased towards the red pa For a color temperature of 5300 K, the photon flux had another slight nm, which was in the green-yellow part of the spectrum. A warm whi 2900 K, wherein the photon flux peaked at around 630 nm, which wa part of the spectrum. The flux gradually decreased as the wavelength the blue part of the spectrum.

Results and Discussions
This section presents simulation results for the behavior of stan all-PSCs under different conditions. The first step involved investigati color temperature of indoor white LED light on the PV parameters o influence of intensity was introduced at a temperature of 2900 K. Fo parameters of the top, bottom, and tandem cells are listed and comp performance. Finally, a proposed design is presented to optimize the tandem cell under the influence of white LED lighting.

Impact of LED Color Temperature
To verify the behavior of the solar cells under investigation with r temperature, the indoor performances of the three all-PSC devices w lecting temperatures of 2900, 5300, and 7500 K. Figure 5 displays the top (wide bandgap), bottom (narrow bandgap), and tandem cells. Th the AM1.5G illumination are also shown for comparison. The JSC beha 5a, indicates an enhancement when increasing the temperature for th reason behind this is that the front subcell had a higher propensity

Results and Discussions
This section presents simulation results for the behavior of standalone and tandem all-PSCs under different conditions. The first step involved investigating the impact of the color temperature of indoor white LED light on the PV parameters of the cells. Next, the influence of intensity was introduced at a temperature of 2900 K. For each step, the PV parameters of the top, bottom, and tandem cells are listed and compared to assess their performance. Finally, a proposed design is presented to optimize the performance of the tandem cell under the influence of white LED lighting.

Impact of LED Color Temperature
To verify the behavior of the solar cells under investigation with respect to LED color temperature, the indoor performances of the three all-PSC devices were assessed by selecting temperatures of 2900, 5300, and 7500 K. Figure 5 displays the PV metrics for the top (wide bandgap), bottom (narrow bandgap), and tandem cells. The metrics related to the AM1.5G illumination are also shown for comparison. The J SC behavior, seen in Figure 5a, indicates an enhancement when increasing the temperature for the front subcell. The reason behind this is that the front subcell had a higher propensity to capture photons from the longer end of the visible spectrum (see Figure 3b) as the cutoff wavelength of the front subcell was about 0.8 µm, resulting in optical responses that were more compatible with the spectrum of the white LED (see Figure 4). In addition, the J SC obtained under LED illumination was lower than that obtained under AM1.5G illumination, due to the reduced light intensity of the LED illumination compared to AM1.5G. dent light intensity (see Figure 5a), VOC decreases as well according to Equation (1). Consequently, the values of VOC under AM1.5G are higher than those under LED illumination. The same trend is valid for the FF, except that the values of FF when involving LED are higher than the value of FF for AM1.5G. The overall consequence is that the PCE was nearly independent of the color temperature of the LED light, while it was much higher than the initial value recorded under AM1.5G. For instance, at 2900 K, the obtained PCE was 21.757%, while its value was 10.110% under 1-sun illumination. Furthermore, when considering the rear subcell, it can be inferred from Figure 5a that the JSC increased with temperature. However, the JSC values obtained under indoor lighting conditions were slightly lower than those obtained under AM1.5G, which can be attributed to the optical response of the rear cell that extended to around 950 nm (see Figure  3b). This means that it was more optimized for AM1.5G than for white LED illumination. The same patterns were observed in the VOC and FF of the rear cell as in the front cell. Regarding the trend of V OC for the front subcell (as depicted in Figure 5b), V OC was nearly independent of the color temperature, while its values under the influence of indoor light were slightly lower than that under AM1.5G. The reason for the decline of V OC values under LED illumination than AM1.5G illumination can be explained based on the following approximate analytical expression [34], where the ideality factor and thermal voltage are denoted as n and V T , respectively, while J o is the reverse saturation current density. As J SC decreases with the reduction in the incident light intensity (see Figure 5a), V OC decreases as well according to Equation (1). Consequently, the values of V OC under AM1.5G are higher than those under LED illumination. The same trend is valid for the FF, except that the values of FF when involving LED are higher than the value of FF for AM1.5G. The overall consequence is that the PCE was nearly independent of the color temperature of the LED light, while it was much higher than the initial value recorded under AM1.5G. For instance, at 2900 K, the obtained PCE was 21.757%, while its value was 10.110% under 1-sun illumination. Furthermore, when considering the rear subcell, it can be inferred from Figure 5a that the J SC increased with temperature. However, the J SC values obtained under indoor lighting conditions were slightly lower than those obtained under AM1.5G, which can be attributed to the optical response of the rear cell that extended to around 950 nm (see Figure 3b). This means that it was more optimized for AM1.5G than for white LED illumination. The same patterns were observed in the V OC and FF of the rear cell as in the front cell. Additionally, the PCE slightly increased with temperature, as demonstrated in Figure 5d, with values that were higher than the PCE at AM1.5G (the PCE value at 2900 K was about 23.794%, while the calibrated value was 16.504%). However, the rate of increase in efficiency was lower than that of the front cell.
On the other hand, Figure 5a shows that the situation was different for the tandem cell. Although the J SC increased with color temperature, its values were still significantly lower than the J SC at AM1.5G. This was an expected trend because the tandem cell was not optimized for the current matching point under indoor lighting conditions, resulting in a current mismatch between the two subcells that made up the tandem. Moreover, the V OC trend was similar to that of the two subcells, as it was almost equal to the sum of the individual V OC of each subcell. The FF also exhibited similar behavior to the two subcells, as it was almost independent of LED temperature. However, the values of FF obtained under LED illumination were higher than those obtained under AM1.5G. This can be explained by the fact that as the light intensity decreased, the J SC also decreased, resulting in a lower loss in the FF, as mentioned in REF [35]. Overall, the PCE of the tandem cell slightly decreased with temperature, with values (16.220% at 2900 K) lower than the initial tandem under AM1.5G (17.578%).
To gain insight into the most dominant factor that affects the PCE, one can use the expression [36], where P in is the input power (W/cm 2 ). According to the results presented in Figure 5, it was observed that changes in the FF and V OC were insignificant, whereas variations in the J SC were more prominent. This suggests that J SC , which was directly linked to the optical response of the photoactive layer, was the key factor that had a direct impact on the PCE. Moreover, to demonstrate the relative change in the PCE when applying indoor LED light instead of the AM1.5G illumination, we define a relative percentage factor ∆ζ, where Figure 6 displays ∆ζ against the color temperature for the three cases (top, bottom, and tandem cells). As indicated in the figure, the front cell showed the highest relative PCE percentage, where ∆ζ = 115.2% at 2900 K. Meanwhile, the tandem demonstrated negative values, implying a performance degradation when applied to indoor conditions. and tandem cells). As indicated in the figure, the front c PCE percentage, where ζ Δ = 115.2% at 2900 K. Meanw negative values, implying a performance degradation whe

Impact of LED Intensity
Next, as demonstrated in Figure 7, the changes of the PV metrics as a function of the incident LED light intensity were further explored by varying its input power in the range 57.9 to 2895 µW/cm 2 (which corresponded to illuminance in the range of 200-10,000 lx) at a constant color temperature of 2900 K. As depicted in Figure 7a, the J SC values of the three cells were almost proportional to P in , which was confirmed with the experimental results [37]. This linear relationship indicated that bimolecular charge recombination, a process where two free charge carriers combine and recombine, is effectively suppressed under short-circuit conditions. Additionally, when increasing the input power, the V OC values of the three cells increased over the whole range (see Figure 7b). Meanwhile, the FF values decreased with P in (see Figure 7c). On the other hand, the PCE trend of the three cells was not alike. While the PCE of the back and tandem cells increased with input power, the behavior of the front subcell was different. The PCE for the front cell increased up to a certain limit and then nearly saturated, with a maximum value of 22.52% at P in = 1160 µW/cm 2 . Furthermore, Figure 8 shows ∆ζ against P in for the three cells. As shown in the figure, the front cell exhibited the highest relative PCE percentage. not alike. While the PCE of the back and tandem cells increased with input power, the behavior of the front subcell was different. The PCE for the front cell increased up to a certain limit and then nearly saturated, with a maximum value of 22.52% at Pin = 1160 µW/cm 2 . Furthermore, Figure 8 shows ζ Δ against Pin for the three cells. As shown in the figure, the front cell exhibited the highest relative PCE percentage.

Optimization of Tandem Cells for an Indoor Environment
Based on the previous simulation results, it has been revealed that the

Optimization of Tandem Cells for an Indoor Environment
Based on the previous simulation results, it has been revealed that the tandem cell is not suitable for indoor conditions as it is optimized for the outdoor one-sun environment. Therefore, it should be specifically optimized for indoor applications to be employed in low-intensity circumstances. Thus, in order to enhance the indoor tandem cell capabilities, a current matching condition should be engineered to achieve maximum PCE. For this purpose, we conducted a simulation run in which both top and bottom polymer blend thicknesses were varied concurrently to search for the optimum efficiency. The range of the top thickness (t top ) and bottom thickness (t bot ) was taken to be from 30 to 100 nm and from 100 to 200 nm, respectively. Figure 9 represents the PV metrics of the tandem all-PSC under a warm LED condition (at 2900 K) and 200 lx. It can be observed from Figure 9a that J SC was maximum for t top = 50 nm (which was half the initial value) as long as t bot was higher than 160 nm. The maximum J SC was about 10.66 µA/cm 2 . Moreover, V OC had a local maximum at t top = 50 nm for any value of t bot (see Figure 9b). The situation was different for the FF, as displayed in Figure 9c; however, the minimum available value was 84%. The trends of J SC , V OC , and FF reflected clearly on the PCE shown in Figure 9d, where the maximum PCE occurred for t top = 50 nm, keeping the bottom thickness higher than 160 nm. On the basis of the simulation results, using a t bot of 200 nm resulted in an optimal PCE of approximately 27.80%.
Based on the previous simulation results, it has been revealed that the tandem cell is not suitable for indoor conditions as it is optimized for the outdoor one-sun environment. Therefore, it should be specifically optimized for indoor applications to be employed in low-intensity circumstances. Thus, in order to enhance the indoor tandem cell capabilities, a current matching condition should be engineered to achieve maximum PCE. For this purpose, we conducted a simulation run in which both top and bottom polymer blend thicknesses were varied concurrently to search for the optimum efficiency. The range of the top thickness (ttop) and bottom thickness (tbot) was taken to be from 30 to 100 nm and from 100 to 200 nm, respectively. Figure 9 represents the PV metrics of the tandem all-PSC under a warm LED condition (at 2900 K) and 200 lx. It can be observed from Figure 9a that JSC was maximum for ttop = 50 nm (which was half the initial value) as long as tbot was higher than 160 nm. The maximum JSC was about 10.66 µA/cm 2 . Moreover, VOC had a local maximum at ttop = 50 nm for any value of tbot (see Figure 9b). The situation was different for the FF, as displayed in Figure 9c; however, the minimum available value was 84%. The trends of JSC, VOC, and FF reflected clearly on the PCE shown in Figure 9d, where the maximum PCE occurred for ttop = 50 nm, keeping the bottom thickness higher than 160 nm. On the basis of the simulation results, using a tbot of 200 nm resulted in an optimal PCE of approximately 27.80%.
Thus, to obtain higher PCE values under white LED lighting, it is recommended to increase the bottom thickness while decreasing the top thickness. It should be mentioned that increasing the thickness of the bottom layer will result in a higher PCE, but this comes at the price of increased processing complexity and costs. These interesting findings highlight the importance of optimizing the thicknesses of both the top and bottom cells in order to enable the tandem cell to perform effectively under indoor conditions.

Conclusions
In this TCAD simulation study, the performance of all-PSCs based on two complementary polymer blends, PM7:PIDT and PM6:PYIT, as well as the tandem produced from integrating them, was assessed. The two standalone PSCs were evaluated under both outdoor (AM1.5G) and indoor light (white GaN LED) environments. Additionally, the tandem cell was evaluated under the same criteria to provide a fair comparison. On the basis Thus, to obtain higher PCE values under white LED lighting, it is recommended to increase the bottom thickness while decreasing the top thickness. It should be mentioned that increasing the thickness of the bottom layer will result in a higher PCE, but this